1. Field of the Invention
The present invention relates generally to amplifiers. The present invention relates more particularly to feed forward amplifiers and methods for aligning feed forward amplifiers.
2. Description of the Prior Art and Related Information
Radio frequency power amplifiers are commonly used in various applications. Examples of such applications include broadcasting, satellite communications, and cellular communications. Radio frequency power amplifiers increase the power of a radio frequency signal so that it is suitable for transmission through the air.
Generally, it is desirable to constrain transmitted radio frequencies utilized in such applications to a desired bandwidth. However, radio frequency power amplifiers inherently distort transmitted signals in a manner that causes them to extend beyond the desired bandwidth. That is, amplifier non-linearities and other factors tend to add components to a transmitted radio signal. These components often have frequencies other than the intended carrier frequency and thus may extend beyond the desired bandwidth.
Such undesirable frequency components waste energy, interfere with other radio signals and may violate Federal Communications Commission (FCC) rules.
Feed forward radio frequency power amplifiers utilize feed forward loops to mitigate such undesirable frequency components and thereby attempt to maintain the entire transmitted radio frequency signal within the desired bandwidth. The undesirable frequency components are mitigated by forming an error signal which tends to cancel them from the amplified signal when the error signal is combined with the amplified signal.
In order for such cancellation to be effective, the feed forward loops must be accurately aligned. According to contemporary practice, alignment of the feed forward loops requires that a feed forward radio frequency power amplifier have at least one breakpoint. Breakpoints prevent radio frequency signals from one part of a feed forward circuit from interfering with the testing of another part of the feed forward circuit.
However, there are several inherent disadvantages associated with the use of breakpoints in feed forward radio frequency power amplifiers. For example, breakpoints tend to introduce undesirable losses into a radio frequency circuit, add undesirably to the cost of the radio frequency power amplifier, and inhibit full automation of feed forward loop alignment.
As those skilled in the art will appreciate, the use of a breakpoint in a radio frequency circuit introduces inherent losses associated with the resistance, capacitance and inductance of the connectors and other components thereof. Indeed, such connectors are also subject to malfunction and thus affect the reliability of the radio frequency power amplifier.
Of course, the addition of such components of the breakpoints as connectors and cables increases the cost of manufacturing the radio frequency power amplifier. This is due not only to the increased cost reflected in the bill of materials, but is also due to such factors as increased assembly costs and reduce yield.
Full automation of a radio frequency power amplifier is inhibited because such breakpoints must be manually opened and closed by a human attendant. The use of automatic equipment to open and close breakpoints is difficult, unreliable and expensive.
As such, although the prior art has recognized, to a limited extent, the problem of aligning feed forward radio frequency power amplifiers, the proposed solutions have, to date, been ineffective in providing a satisfactory remedy. Moreover, although such contemporary feed forward radio frequency power amplifiers have proven generally suitable for their intended purposes, they possess inherent deficiencies which detract from their overall effectiveness and desirability. Therefore, it is desirable to provide a feed forward radio frequency power amplifier which does not have breakpoints and which can be aligned via a fully automated process.